It is a problem to create a structure absorbing energy at oblique impacts generating tangential force components, for example an impact between a person and a moving object or surface. The structure may for example be a helmet, a protective clothing or other force absorbing structures.
In prior art there are presented a number of solutions comprising at least a first and a second layer or part which are slidably moveable in relation to each other in order to absorb an impact force. In order to function properly the layers are connected by one or several connecting arrangements.
In one embodiment the structure is a helmet. Most helmets comprises a hard outer shell, often made of a plastic or a composite material, and an energy absorbing layer, called a liner, of energy absorbing material. Nowadays, a protective helmet has to be designed so as to satisfy certain legal requirements which relate to inter alia the maximum acceleration that may occur in the center of gravity of the head at a specified load. Typically, tests are performed, in which what is known as a dummy skull equipped with a helmet is subjected to a radial blow towards the head. This has resulted in modern helmets having good energy-absorption capacity in the case of blows radially against the skull while the energy absorption for other load directions is not as optimal.
In the case of a radial impact the head will be accelerated in a translational motion resulting in a translational acceleration. The translational acceleration can result in fractures of the skull and/or pressure or abrasion injuries of the brain tissue. However, according to injury statistics, pure radial impacts are rare.
On the other hand, a pure tangential hit that result in a pure angular acceleration to the head are rare, too.
The most common type of impact is oblique impact that is a combination of a radial and a tangential force acting at the same time to the head. The oblique impact results in both translational acceleration and angular acceleration of the brain. Angular acceleration causes the brain to rotate within the skull, creating injuries on bodily elements connecting the brain to the skull and also to the brain itself.
Examples of rotational injuries are on the one hand subdural haematomas, SH, bleeding as a consequence of blood vessels rupturing, and on the other hand diffuse axonal injuries, DAI, which can be summarized as nerve fibers being over stretched as a consequence of high shear deformations in the brain tissue. Depending on the characteristics of the rotational force, such as the duration, amplitude and rate of increase, either SH or DAI occur, or a combination of these is suffered. Generally speaking, SH occur in the case of short duration and great amplitude, while DAI occur in the case of longer and more widespread acceleration loads. It is important that these phenomena are taken into account so as to make it possible to provide good protection for the skull and brain.
The head has natural protective systems adapted to dampen these forces using the scalp, the hard skull and the cerebrospinal fluid between the skull and the brain. During an impact, the scalp and the cerebrospinal fluid acts as rotational shock absorber by both compressing and sliding over and under the skull, respectively. Most helmets used today provide no protection against rotational injury.
In the applicant's prior applications WO2011139224A1 and EP1246548B1 it is described a helmet comprising a first and a second helmet part slidably arranged in relation to each other to protect against rotational injury. The first helmet part is arranged closer to a wearers head and the second part is arranged radially outside the first helmet part.
Further it is in WO2011139224A1 and EP1246548B1 described several ways of connecting the first helmet part with the second helmet part. The connecting arrangements are arranged to absorb energy by deforming in an elastic, semi-elastic or plastic way when large enough strain are applied to the outer helmet part.
When using these connection arrangements it is difficult to control the motion between the first and second part and thus also the force absorption curve.